1. Field of the Invention
This invention relates to a method for the preparation of a cathode active material, capable of reversibly doping/undoping lithium, and to a method for the preparation of a non-aqueous electrolyte cell employing this cathode active material.
2. Description of Related Art
Nowadays, in keeping up with the recent marked progress in the electronic equipment, researches into re-chargeable secondary cells, as power sources usable conveniently and economically for prolonged time, are underway. Representative of the secondary cells are lead accumulators, alkali accumulators and non-aqueous electrolyte secondary cells.
Of the above secondary cells, lithium ion secondary cells, as non-aqueous electrolyte secondary cells, have such merits as high output and high energy density. The lithium ion secondary cells are made up of a cathode and an anode, including active materials capable of reversibly doping/undoping lithium ions, and a non-aqueous electrolyte.
As the anode active material, metal lithium, lithium alloys, such as Lixe2x80x94Al alloys, electrically conductive high molecular materials, such as polyacetylene or polypyrrole, doped with lithium, inter-layer compounds, having lithium ions captured into crystal lattices, or carbon materials, are routinely used. As the electrolytic solutions, the solutions obtained on dissolving lithium salts in non-protonic organic solvents, are used.
As the cathode active materials, metal oxides or sulfides, or polymers, such as TiS2, MoS2, NbSe2 or V2O5, are used. The discharging reaction of the non-aqueous electrolyte secondary cells, employing these materials, proceeds as lithium ions are eluated into the electrolytic solution in the anode, whilst lithium ions are intercalated into the space between the layers of the cathode active material. In charging, a reaction which is the reverse of the above-described reaction proceeds, such that lithium is intercalated in the cathode. That is, the process of charging/discharging occurs repeatedly by the repetition of the reaction in which lithium ions from the anode make an entrance into and exit from the cathode active material.
As the cathode active materials for the lithium ion secondary cells, LiCoO2, LiNiO2 and LiMn2O4, for example, having a high energy density and a high voltage, are currently used. However, these cathode active materials containing metallic elements having low Clarke number in the composition thereof, are expensive, while suffering from supply difficulties. Moreover, these cathode active materials are relatively high in toxicity and detrimental to environment. For this reason, novel cathode active materials, usable in place of these materials, are searched.
On the other hand, it is proposed to use LiFePO4, having an olivinic structure, as a cathode active material for the lithium ion secondary cells. LiFePO4 has a high volumetric density of 3.6 g/cm3 and is able to develop a high potential of 3.4 V, with the theoretical capacity being as high as 170 mAh/g. In addition, LiFePO4 in an initial state has an electro-chemically undopable Li at a rate of one Li atom per each Fe atom, and hence is a promising material as a cathode active material for the lithium ion secondary cell. Moreover, since LiFePO4 includes iron, as an inexpensive material rich in supply as natural resources, it is lower in cost than LiCoO2, LiNiO2 or LiMn2O4, mentioned above, while being more amenable to environment because of lower toxicity.
However, LiFePO4 is low in electronic conduction rate, such that, if this material is used as a cathode active material, the internal resistance in the cell tends to be increased. The result is that the polarization potential on cell circuit closure is increased due to increased internal resistance of the cell to decrease the cell capacity. Moreover, since the true density of LiFePO4 is lower than that of the conventional cathode material, the charging ratio of the active material cannot be increased sufficiently if LiFePO4 is used as the cathode active material, such that the energy density of the cell cannot be increased sufficiently.
So, a proposal has been made to use a composite material of a carbon material and a compound of an olivinic structure having the general formula of LixFePO4 where 0 less than xxe2x89xa61, referred to below as LiFePO4 carbon composite material, as a cathode active material.
As a method for the preparation of an LiFePO4 carbon composite material having such olivinic structure, there is proposed such a method consisting in mixing lithium phosphate Li3PO4, Fe3(PO4)2 or its hydrates Fe3(PO4)2.nH2O, where n denotes the number of hydrates, as starting material for synthesis, adding carbon to the resulting mixture and sintering the resulting product at a preset temperature.
For producing the LiFePO4 carbon composite material, the sintering temperature for the mixture needs to be higher than the temperature for which surface activity of the starting materials for synthesis can be exhibited. Since the melting point of Li3PO4, Fe3(PO4)2 or its hydrates Fe3(PO4)2.nH2O, where n denotes the number of hydrates, as starting material for synthesis of LixFePO4, is 800xc2x0 C. or higher, sufficient surface activity of the starting materials for synthesis can be manifested by setting the sintering temperature to 800xc2x0 C. or higher. However, if the sintering temperature is high, the energy consumption required for synthesis is increased, as a result of which the production cost is raised. Moreover, a higher sintering temperature means an increased load on e.g., a unit for synthetic reaction, and hence is not desirable for mass production.
Therefore, the sintering temperature of approximately 600xc2x0 C. in synthesizing the LiFePO4 carbon composite material is most desirable for the performance of the cathode active material. However, the particle size of the starting materials for synthesis of the LiFePO4 carbon composite material is usually 2 to 10 xcexcm. If the sintering temperature is 600xc2x0 C., sufficient surface activity cannot be developed in sintering on the surface of the starting material for synthesis having this particle size. The result is that difficulties are encountered in achieving single-phase synthesis of the LiFePO4 carbon composite material.
In view of the above depicted status of the art, it is an object of the present invention to provide a method for the preparation of a cathode active material by means of which the reaction efficiency in sintering can be improved such that single-phase synthesis of the LiFePO4 carbon composite material can be attained even at a sintering temperature of 600xc2x0 C. lower than the melting point of the starting materials for synthesis thereby realizing superior cell characteristics.
It is another object of the present invention to provide a method for the preparation of the non-aqueous electrolyte cell having superior cell characteristics such as cell capacity or cyclic characteristics through the use of the so-produced LiFePO4 carbon composite material as the cathode active material.
In one aspect, the present invention provides a method for the preparation of a cathode active material comprising:
mixing, milling and sintering a starting material for synthesis of a compound represented by the general formula LixFePO4, where 0 less than xxe2x89xa61, and adding a carbon material to the resulting mass at an optional time point in the course of said mixing, milling and sintering;
employing Li3PO4, Fe3(PO4)2 or hydrates Fe3(PO4)2.nH2O thereof, where n denotes the number of hydrates, as the material for synthesis of said LixFePO4; and
setting the particle size distribution of particles of the starting material for synthesis following the milling with the particle size not less than 3 xcexcm to 22% or less in terms of the volumetric integration frequency.
Since the particle size distribution of the milled starting materials for synthesis is defined as described above, the surface area of the starting materials for synthesis at the time of sintering is sufficient to develop sufficient surface activity. Thus, if the mixture is sintered at a temperature not higher than the melting point of the starting materials for synthesis, single-phase synthesis of the LiFePO4 carbon composite material may occur satisfactorily. It is noted that milling denotes comminuting and mixing simultaneously.
In another aspect, the present invention provides a method for the preparation of a non-aqueous electrolyte cell including a cathode having a cathode active material, an anode having an anode active material and a non-aqueous electrolyte, wherein
in preparing said cathode active material, a starting material for synthesis of a compound represented by the general formula LixFePO4, where 0 less than xxe2x89xa61, is sintered, mixed, milled and a carbon material is added to the resulting mass at an optional time point in the course of said mixing, milling and sintering;
Li3PO4, Fe3(PO4)2 or hydrates Fe3(PO4)2.nH2O thereof, where n denotes the number of hydrates, are used as the starting material for synthesis of said LixFePO4; and
the particle size distribution of particles of the starting materials for synthesis following the milling with the particle size not less than 3 xcexcm is set to 22% or less in terms of the volumetric integration frequency.
Since the particle size distribution of the milled starting material for synthesis is defined as described above, the surface area of the starting material for synthesis at the time of sintering is sufficient to develop sufficient surface activity. Thus, if the mixture is sintered at a temperature not higher than the melting point of the starting materials for synthesis, single-phase synthesis of the LiFePO4 carbon composite material may occur satisfactorily. By employing this composite material as a cathode active material, a non-aqueous electrolyte cell having superior cell characteristics may be produced.